The application of standard corrosion modelling systems for carbon steel (CS) for a number of highly sour oil & gas projects has predicted corrosion rates ranging from 10 to 35 mm/yr which drives material selection to corrosion resistant alloys (CRAs). This is the case even where corrosion inhibitors are considered with overall effectiveness in the range 90 to 95% (where effectiveness = efficiency x application time). In order to validate the model predictions a series of laboratory corrosion tests were carried out using a CS pipeline material to establish whether the natural formation of protective iron sulfide corrosion products can be demonstrated to provide satisfactory protection. If this was the case then it would enable CS to be considered either with or without corrosion inhibition and would avoid the need to use expensive CRAs and could result in cost savings of hundreds of millions of US dollars.

The paper describes short term studies to assess the feasibility of this approach. Tests have been conducted under simulated dynamic production environment conditions ranging in temperature from 50 to 210°C with partial pressures of H2S up to 11 bara. Both formation and condensed water systems have been simulated.

The degree of protection afforded by scales formed in these conditions has been characterised by electrochemical evaluation and by gravimetric analysis of coupons from Rotating Cylinder Electrode (RCE) and rotating cage apparatus. The morphology of the scale has been studied under the Scanning Electron Microscope (SEM) and analysed by Energy Dispersive X-rays (EDX) and X-ray Diffraction (XRD. Natural corrosion rates from these short term tests are significantly lower than predicted with measured rates of between 0.04 to 2.2 mm/yr (i.e. laboratory values ranging from 16 to 900 times lower than predicted). This laboratory data indicates that CS is potentially suitable for field service and further selective long term tests with and without corrosion inhibitor are planned to confirm these results.

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